Douglas McCain was 33-years-old, an American citizen raised in Minnesota and until recently living in San Diego. But this week, U.S. officials confirmed that he was killed in Syria in combat, fighting on the side of the so-called Islamic State or ISIS. McCain is one of many young men from the U.S. and Western Europe who have made the journey to Syria to join the fight. Yesterday, British Prime Minister David Cameron said that at least 500 people may have left Britain to fight in Syria and says it's likely that the man who beheaded journalist James Foley made the same trip.

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DAVID CAMERON: We need to do more to stop people traveling, to stop those who do go from returning and to deal decisively with those who are already here.

RATH: Jessica Stern studies terrorism. And she says the civil war in Syria has been uniquely appealing to Westerners.

JESSICA STERN: We've never seen this many Westerners traveling abroad to join a so-called jihad.

RATH: Why do you think that is? What's motivating these young people to travel halfway around the world to fight in a war they're not really directly connected with?

STERN: I think there are lots of reasons. One is that the group is incredibly skilled at making this jihad look like a very exciting adventure. They're using social media in a way we've never seen a group use it before. They're flaunting their successes. They're claiming, come on in - it's so easy to get here. If you don't have big muscles, well, maybe you can help us with Internet requirements. Maybe you're a doctor. Maybe you can send money. And they show pictures of what it's like to live in their caliphate with nice streets and cars and houses. So it definitely looks a lot more appealing than going to Afghanistan.

RATH: Well, they're flaunting their success in social media but they're also flaunting their brutality. I mean, just really horrific images and video that have come out. Why wouldn't that turn off young people, you know, again in Western countries?

STERN: Well, I'm sure that it does turn off most young people. But at the same time clearly there are some people who really feel that their fellow Muslims are in desperate need and that this kind of violence, which is a violation of every religious tradition - under these conditions, they claim, is justified. So some people - thrill seekers, people who want a violent adventure - this seems to be a very effective recruitment tool. Obviously, you know, it's disgusting for most of us.

RATH: But it sounds like these are not necessarily true believers - people who are deep into the Quran necessarily - you use the term thrill-seeker.

STERN: It appears that a surprising number of converts are being drawn into this jihad and also people who are relatively ignorant about Islam. And we have the recent case of a couple of British citizens who, before going to Syria, had ordered "Islam For Dummies" and "The Quran For Dummies." So clearly these are not people who are Islamic scholars. It seems to be people who are seeking belonging to something bigger than themselves. And suddenly there's a global caliphate that they can join, whether they're converts or Muslims who are sort of born again, but born again into a very distorted version of Islam.

RATH: That's Jessica Stern. She's a fellow at the FXB Center for Health and Human Rights at Harvard and the author of "Terror In The Name Of God: Why Religious Militants Kill." Jessica, thanks very much.